1. Field of the Disclosure
The disclosure relates to digital telephone communications.
2. Introduction
In a digital telephonic communication system, it is frequently desirable to be able to rapidly switch between different channel rates in order to control network congestion. Parametric vocoders generally have a much lower rate (and somewhat lower voice quality) than speech-specific waveform coders, so a switch to the lower rate coder is desirable when network congestion is building. Conversely, a switch to the higher rate coder is warranted when the network is lightly loaded. These switches may be initiated quickly at the transmitter, with no advance warning to the receiver.
There are two problems with making a changeover between the coders. (1) The output waveforms of the two coding algorithms will not match. This is true because the waveform-preserving decoder will seek to preserve the actual waveform, while the parametric vocoder decoder will only preserve the salient features gross spectrum, pitch, voicing and signal level). This problem occurs with switches in either direction. (2) The parametric vocoder may require several frames of valid data before it starts to output a signal. This is especially true with TDVC, which has two layers of memory in the decoder (a 3-deep parameter buffer and a 2-frame interpolation buffer). So if an abrupt changeover from the waveform coder to the vocoder occurs, there could be up to three frames of zero-valued (or low-amplitude) output signal before the synthesizer is completely ramped up.
Finally, one other problem may be experienced when changing abruptly to TDVC mode when using some implementation platforms. Due to the interaction of the processor and operating system, some systems will perform arithmetic exception processing when low amplitude signals (e.g. underflow conditions) are processed in the TDVC speech synthesizer. This situation will occur at changeover during TDVC's startup, and must be avoided, since it slows down the processing as much as 5000%.